The present invention relates to a phase difference detector for use in, for example, an automatic focus detector for a camera. More particularly, the present invention relates to a phase difference detector designed to execute arithmetic processing for phase difference detection by an analog signal processing method.
A typical conventional automatic focus detector for a camera has heretofore been arranged as shown in FIG. 20. More specifically, a condenser lens 3, a separator lens 4 and a phase difference detector are disposed to the rear of a film equivalent plane 2 which is in turn positioned to the rear of a photographic lens 1.
The phase difference detector comprises line sensors 5, 6 defined by CCDs or the like which receive rays of light carrying a pair of images formed by the separator lens 4. The CCDs subject the received light to photoelectric conversion, and a processing circuit 7 distinguishes a focused state on the basis of signals generated in the picture elements of the line sensors 5, 6 in accordance with luminance intensity distributions.
When the subject image is focused forwardly of the film equivalent plane 2, that is, in the case of front mis-focus, the images formed on the line sensor 5, 6 are close to the optical axis 8, whereas, in the case of rear mis-focus, said images are remote from the optical axis 8. When the subject image is correctly focused, the images formed on the line sensors 5, 6 are at predetermined positions intermediate those in the front mis-focus and the rear mis-focus conditions. Accordingly, the processing circuit 7 detects the positions of the formed images relative to the optical axis 8 to thereby distinguish the focused state.
The phase difference detection technique is employed to detect the positions of the images formed on the line sensors 5, 6. According to this technique, a correlation value of a pair of images formed on the line sensors 5, 6 is obtained by an arithmetic operation based on the following formula (1), and a focused state is distinguished on the basis of the amount of relative movement (phase difference) of these images required until the correlation values reaches a minimum: ##EQU1## where l is an integer having a value of 1 to 9 and represents the above-described amount of relative movement.
For example, B(k) represents signals which are respectively output from the picture elements of the line sensor 5 in a time serial manner, while R(k+l-1) represents signals respectively output from the picture elements of the line sensor 6 in a time serial manner, and if the arithmetic processing of the above-described formula (1) is the executed every time l is changed from 1 to 9, correlation values H(1), H(2) . . . H(9) are obtained. The focused state is detected when, for example, the correlation value H(5) is at a minimum value, and if a correlation value at a position which is offset from the focused position is at a minimum value, the amount of offset, that is, the phase difference corresponding to l=5, can be detected as the amount by which the subject image is out of focus.
The arrangement of the conventional processing circuit 7 is shown in FIG. 21. Analog electric signals B(k) and R(k) respectively generated from the picture elements of the line sensors 5, 6 are converted into, for example, 8-bit digital data, by means of an A/D converter 9, and the digital data is temporarily stored in a RAM (Random Access Memory) 11 through a microcomputer 10. Thereafter, the arithmetic processing of the above-described formula (1) is executed on the basis of the stored digital data.
The conventional phase difference detector of the type described above suffers, however, from the following problems. Since the arithmetic operation is executed by digital signal processing using a microcomputer or the like, a costly A/D converter or the like is needed in order to effect a high-speed and high-precision arithmetic operation. In addition, there will be produced a rounding error attributable to the restriction on the number of quantizers and the like which perform arithmetic operations, and this leads to a lowering in the degree of accuracy of the arithmetic operation. Further, the work which is needed to design a computer program for the arithmetic process is increased, and it is necessary to provide a memory for storing a large amount of digital data. For these reasons, the number of required parts is increased, and the size of the apparatus is thus enlarged, disadvantageously.